1. Field of the Invention
The present invention relates to a flow rate sensor which is usually employed to measure an intake air flow rate in an internal combustion engine, particularly relates to a flow rate sensor which is used to measure the flow rate of a fluid on the basis of a heat transfer phenomenon where a heat is transferred either from a heating element or from a portion heated by the heating element to the fluid.
2. Description of the Related Art
Japanese Unexamined Patent Publication No.8-313318 has disclosed a thermo-sensitive type flow rate sensor which is used to measure the flow rate of a fluid flowing through a fluid passage, on the basis of a heat transfer phenomenon where a heat is transferred either from a heating element or from a portion heated by the heating element to the fluid.
FIG. 20 is a front view of the conventional thermo-sensitive type flow rate sensor disclosed in Japanese Unexamined Patent Publication No.8-313318. FIG. 21 is a cross sectional view of the thermo-sensitive type flow rate sensor of FIG. 20.
Referring to FIG. 21, an inner duct 22, which is used as a detecting pipe conduit and is formed with a bell mouth portion, is located within a fluid passage 6 through which a fluid (whose flow rate is to be measured) flows. The fluid flows from the left to the right in the drawing through the fluid passage 6, and a flow rate detecting element 20 is disposed within the inner duct 22.
The flow rate detecting element 20 is comprised of a ceramic substrate and a platinum layer formed by depositing platinum as served a thermo-sensitive electrically resistant material on the surface of the ceramic substrate. The thermo-sensitive electrically resistant material has a property whereby the electric resistance thereof will change with changes in temperature. Further, the platinum layer is formed into a meander pattern so as to serve as a flow rate detecting resistance 24. Moreover, a fluid temperature compensating resistance 21 is also made of a platinum which is a thermo-sensitive electrically resistant material, and is disposed upstream of the inner duct 22. A fluid rectifying means 23 is made of a resin and is formed into a honeycomb structure. Such fluid rectifying means 23 is positioned upstream of the fluid temperature compensating resistance 21.
An electronic circuit case 8 accommodating an electronic circuit board 7 is provided on the outside of the fluid passage 6. Mounted and fixed on the electric circuit board 7 is an electronic circuit for detecting the flow rate of a flowing fluid. In practice, the electronic circuit is electrically connected with both the flow rate detecting resistance 24 and the fluid temperature compensating resistance 21.
Referring now to FIG. 20, on one side of the circuit case 8 there is provided a connector 11 which is used to supply an electric power from the outside to the flow rate sensor, and to obtain a flow rate signal from the flow rate sensor so as to send the flow rate signal to a predetermined place outside the fluid passage 6.
In use of such conventional thermo-sensitive type flow rate sensor 25, an electric current flowing into the flow rate detecting resistance 24 of the flow rate detecting element 20, is controlled by the electronic circuit attached on the board 7, in a manner such that an average temperature of the flow rate detecting resistance 24 will rise to a predetermined value which is 200.degree. C. higher than a fluid temperature detected by the fluid temperature compensating resistance 21. In more detail, when a flowing fluid quantity is small, an amount of heat transferred from the flow rate detecting resistance 24 to the flowing fluid will also be small, thus an electric current necessary for heating will decrease. On the other hand, when a flowing fluid quantity is large, an amount of heat transferred from the flow rate detecting resistance 24 to the flowing fluid will also be large, thus an electric current necessary for heating will be increased. Thus, in the thermo-sensitive type flow rate sensor 25, an electric current for heating is detected and used as a fluid rate signal, thereby detecting an actual flow rate of a fluid flowing through the passage 6 having a predetermined cross section area.
The thermo-sensitive type flow rate sensor 25, which is constructed in the above mentioned manner, is often used as an intake air flow rate sensor for an automobile engine, as shown in FIG. 22. Referring to FIG. 22, the flow rate sensor 25 is positioned within an intake air pipe 28 which is located downstream of an air cleaner element 27 enclosed in an air cleaner case 26. The air cleaner element 27 is a filter means made of a non-woven fabric or a filter paper, which is used to capture the dust entrained in the intake air so as to prevent it from entering the engine. Accordingly, after an automobile travels for a while, the air cleaner element 27 will get blocked due to the dust. As a result, there will be a change in the flow speed distribution of a fluid on the downstream side of the air cleaner element 27 before the fluid arrives at the flow rate sensor 25.
In fact, the flow rate detecting element 20 of the flow rate sensor 25 can detect only a part of the fluid flowing through the entire cross section of the fluid passage 6. Accordingly, although the total quantity of a fluid flowing through the passage 6 does not change, a change in the flow speed distribution of a fluid on the upstream side of the flow rate sensor 25, will bring about an error to a flow rate detecting result.
In order to solve the above problem, it has been suggested that a fluid rectifying means 23 be provided in the fluid passage 6 upstream of the flow rate sensor 25, as shown in FIGS. 20 and 21. To obtain a sufficient rectifying effect, such kind of fluid rectifying means 23 should be made so that the holes formed therethrough are quite small. However, since the fluid rectifying means 23 has a honeycomb structure and since such means is required to have a sufficient rigidity, it is difficult to manufacture the fluid rectifying means 23 with a small thickness. As a result, a finally obtained fluid rectifying means 23 has only a small aperture ratio (a small aperture area). Moreover, the use of such fluid rectifying means 23 will induce a lot of small eddy flows on the downstream side thereof, thus producing a large fluid flow resistance around the flow rate sensor 25. It is understood that if the fluid flow resistance on the flow rate sensor 25 is large, an amount of intake air to be supplied to an automobile engine will be small, resulting in a problem that the automobile engine can only produce a small output power.
Another conventional flow rate sensor has been disclosed in Japanese Unexamined Patent Publication No.7-71985. In order to obtain a sufficient fluid rectifying effect, this conventional flow rate sensor employs a fluid rectifying means combined a honeycomb structure made of a resin and a net-like grating structure. This however presents another problem, that is, a fluid flow resistance is caused not only due to the honeycomb structure but also due to the net-like grating structure.
On the other hand, in the conventional flow rate sensor 25 shown in FIGS. 20 and 21, the inner duct 22 is formed to have a bell mouth portion in order to obtain a uniform flow speed distribution. However, since a fluid cracking phenomenon will occur in an outer circumferential portion of the upstream end of the inner duct 22, there is also a large fluid flow resistance existing on the flow rate sensor 25.
A further example representing a conventional flow rate sensor is disclosed in Japanese Unexamined Patent Publication No.8-5430. According to this prior art, a central member having a by-pass structure is disposed in the fluid flow passage. Since such central member is provided with a complex by-pass structure, there are too many items and parts necessary to be installed in the fluid flow passage. Because such by-pass structure is prone to produce a fluid leakage, it is required that a particular attention be paid when assembling various items and parts into the fluid flow passage in order to prevent a possible leakage. This, however, results in a low productivity for the assembling operation.